1. Field of the Invention
The present invention relates to an image display device and a method of displaying an image in which the qualities of moving image and still image are improved while controlling increase in power consumption.
2. Description of the Related Art
In recent years, offering of technical advantages to flat panel display devices, such as liquid crystal display devices or organic electro luminescence (EL) display devices, are in progress, and such display devices are becoming widely used in the field of TV-set in which cathode ray tubes (hereinafter referred to as CRT) have been mainly used.
However, the liquid crystal display devices or the organic EL display devices have a problem such that images are percieved to be out-of-focus when moving images are displayed. This problem is caused by the fact that time-axis characteristics in method of displaying image are different between the liquid crystal display device or the organic EL display device and the CRT. The cause of this problem will be described briefly below.
The liquid crystal display device or the organic EL display device using a transistor as a change-over switch for switching between display and non-display for each pixel is a display device employing a display method in which a displayed image is maintained for one frame period corresponding to one frame (hereinafter referred to as hold-type display). On the other hand, the CRT is a display device employing a display method in which each pixel is turned on for certain period and then is darkened (hereinafter referred to as impulse-type display).
In the case of the hold-type display, a state in which the same image is kept displayed from the timing where one frame is displayed to the timing where the next frame is displayed in the moving image is resulted. From the timing where a frame N in the moving image is displayed until the timing where a next frame N+1 is displayed (between frames), the same image as in the frame N is kept displayed. When a moving object is shown in the moving image, the moving object stands still from the timing where the frame N is displayed until the timing where the frame N+1 is displayed on a screen. When the frame N+1 is displayed, the moving object moves discontinuously.
On the other hand, when the observer pays attention to the moving object and keeps observing while following the movement of the moving object (when the ocular movement of the observer is the following movement), the observer moves his/her eyes and tries to follow the moving object with continuous smoothness without consciousness.
Then, there arises a difference between the movement of the moving object on the screen and the movement of the moving object which the observer expects. Due to this difference, a shifted image is percieved to retinas of the observer according to the speed of the moving object. Since the observer perceives a shift image on which the shifted image is superimposed, he/she gets the impression that the moving image is out-of-focus.
The higher the speed of the moving image, the larger shift amount of the image is percieved on his/her retinas, and hence the observer gets the stronger impression of out-of-focus.
In the case of the impulse-type display, such “out-of-focus” does not occur. It is because black color is displayed between the frames (for example, between the frame N and the frame N+1 described above) of the moving image in the case of the impulse-type display.
By displaying black between the frames, even when the observer moves his/her eyes to follow the moving object smoothly, the observer is not viewing the image other than the moment when the image is displayed. Since the observer recognizes each frame of the moving image as an independent image, the image percieved on his/her retinas is never be shifted.
In order to solve the above-described problem in the display device in which the hold-type display is performed, a technique to display “black” by any means after having displayed a frame (for example, see Japanese Patent Publication (KOKAI) No. JP-A-11-109921) is proposed.
There is also proposed a technique to determine whether the input image is moving image or still image and only when it is the moving image, display black between the consecutive frames (For example, see Japanese Patent Publication (KOKAI) No. JP-A-2002-123223).
In JP-A-11-109921, by providing “black” screen intentionally between the frames on the liquid crystal display, a quasi-impulse-type display as the CRT is created to restrain deterioration of the image quality of the moving image. However, a power consumed by a backlight which is continuously turned on during black display is wasted. Also, in the case of the still image, there arises such problem that flicker which is caused by the impulse-type display may occur.
In JP-A-2002-123223, in order to solve the above-described problem, it is controlled in such a manner that the hold-type display is employed for the still image display and the impulse-type display is employed for the moving image display. However, in the above-described method, for example, black is displayed in the same manner for the moving image with small motion and for the moving image with large motion, sufficient effect for reducing power consumption cannot be expected. Although the criteria between the moving image and the still image may be set to a value rather closer to the moving image in order to increase the effect for reducing power consumption, the quality of the moving image is deteriorated in this case. In addition, the abrupt change in a black display time ratio (black display period/one frame period) such as the change-over between the impulse display and the hold display are recognized as the flicker by the observers, which may result in deterioration of the image quality.